Taking on Extreme Angles
Some projects present us with creating half lap dados with extreme angles. Your first thought may be, "...no worries, I'll just use a table saw...". A potential challenge could be that your table saw will not easily support pushing wood through the blade at such angles with any precision.
The solution? Use a portable router with a guide to make the cuts at the outer boundaries of the dado. We will call these your money cuts.
Install two screws in the router base and advance them deep enough underneath the base, in this case about 1/2". Press the router up against the guide so the two screws touch the edge of the guide. Measure the distance between the guide and cutting edge of the router bit. In this example, it was coincidently 1-inch. Use a ruler or tape to measure 1-inch away from the desired "money cut" edge in two separate locations. Place your guide edge along these two reference points. Clamp in place. Drag your router along this guide at your desired depth throughout your cut length. Repeat this procedure on the other side of the dado. Now you have two parallel dado cuts opposite each other.
Take your piece to the table saw and dado the remaining portion of the dado between the two money cuts. Pay special attention to keep the dado saw blade away from your two new router-created cuts throughout the dado process.
Viola' ... precise fit dado!